Journal Archive

Johnson Matthey Technol. Rev., 2021, 65, (3), 418
doi: 10.1595/205651320X15940360546454

Ultrasonic and Thermophysical Studies of Ethylene Glycol Nanofluids Containing Titania Nanoparticles and Their Heat Transfer Enhancements

Next-generation heat transfer nanofluids for industrial applications

  • Mohit Gupta
  • Department of Physics, University of Allahabad, Senate House Campus, University Road, Old Katra, Prayagraj (Allahabad), Uttar Pradesh - 211002, India
  • Devraj Singh
  • Department of Physics, Prof. Rajendra Singh (Rajju Bhaiya) Institute of Physical Sciences for Study and Research, Veer Bahadur Singh Purvanchal University, Jaunpur - 222003, Uttar Pradesh, India
  • Shakti Pratap Singh*
  • Prof. Rajendra Singh (Rajju Bhaiya) Institute of Physical Sciences for Study and Research, Veer Bahadur Singh Purvanchal University, Jaunpur - 222003, Uttar Pradesh, India; Department of Physics, University of Allahabad, Senate House Campus, University Road, Old Katra, Prayagraj (Allahabad), Uttar Pradesh - 211002, India
  • Ashish Mathur
  • Department of Physics, School of Engineering, UPES Bidholi Campus, Dehradun, Uttarakhand - 248007, India
  • Shikha Wadhwa
  • Department of Chemistry, School of Engineering, UPES Bidholi Campus, Dehradun, Uttarakhand - 248007, India
  • Aashit K. Jaiswal
  • Department of Physics, H.N.B. Government P.G. College, Naini, Prayagraj - 211008, India
  • Dharmendra K. Singh
  • Department of Physics, Shivharsh Kisan P.G. College, Civil Lines, Basti, Uttar Pradesh - 272001, India
  • R. R. Yadav
  • Department of Physics, University of Allahabad, Senate House Campus, University Road, Old Katra, Prayagraj (Allahabad), Uttar Pradesh - 211002, India
  • *Email:

Article Synopsis

In the present investigation, TiO2 nanostructures were synthesised via a simple sol-gel technique and characterised with X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX), high-resolution transmission electron microscopy (HR-TEM) and ultraviolet-visible (UV-vis) spectroscopy. The temperature and concentration dependence of thermal conductivity enhancement (TCE) and ultrasonic velocity have been explored in ethylene glycol-based TiO2 nanofluids. The obtained results showed 24% enhancement in thermal conductivity at higher temperature (80°C) of the base fluid ethylene glycol by adding 1.0 wt% of TiO2 nanoparticles. The behaviour of TCE and ultrasonic velocity with temperature in prepared nanofluids has been explained with the help of existing phenomena. The increase in ultrasonic velocity in ethylene glycol with TiO2 nanoparticles shows that a strong cohesive interaction force arises among the nanoparticles and base fluid. These results divulge that TiO2 nanoparticles can be considered for applications in next-generation heat transfer in nanofluids.

1. Introduction

Nanoparticles are traditionally defined as particles with at least one of the characteristic dimensions being up to 100 nm. Nanoparticles have a large surface to volume ratio. This is the most important factor to explain the anomalous behaviour of nanoparticles as compared to their bulk counterparts (13). Metal oxide nanoparticles, predominantly transition metals, are much preferred for their wide and attractive choice of properties (46). The metals with their varying valences can form a vast range of oxide compounds when processed through suitable synthesis methodologies (7, 8). Metal oxides can display metallic, semiconducting or insulating character according to their electronic structure (911). Among the metal oxide nanoparticles, TiO2 nanoparticles are attractive due to their high stability, commercial availability and comparatively low cost (12, 13). They are also free from health hazards (14). TiO2 nanoparticles have also attracted considerable attention for their potential applications in technologies such as fabrication of microelectronic circuits, sensors, fuel cells, solar cells, electronics, piezoelectric devices, medicine, pharmaceuticals, cooling, heat transfer and power generation (1517). The use of oxides in the semiconductor industry is the most active area and generally computer chips are made from oxide compounds.

Intrinsically small thermal conductivity of conventional heat transfer fluids is a primary limitation to developing energy efficient heat transfer fluids for cooling applications. One innovative approach is to suspend low dimensional particles in base fluids to enhance their heat transfer performance (1820). But micrometre or millimetre sized particles cannot be used in microsystems because they can block microchannels, damage or wear out pumps, pipes or bearings and these particles tend to precipitate. Yu and Choi (21) in 1995 first coined the term ‘nanofluids’: a nanoparticle-liquid dispersion consisting of particles with 1–100 nm size offers new potential for heat transfer fluids. Stable nanofluids are prepared mainly by two techniques: (a) single step technique; and (b) two step technique. In the single step technique, nanoparticles are made and dispersed simultaneously into the base fluids. In the two step technique, nanoparticles are prepared first and then dispersed into base fluids. Most nanofluids containing oxide nanoparticles and carbon nanotubes are produced by the two step method. The nanoparticles are dispersed into liquid using an ultrasonic bath or high power tip ultrasonicator with different sonication time while controlling overheating of the nanofluids. In the present investigation, the two step method of nanofluids synthesis has been used to prepare TiO2-ethylene glycol nanofluids (22, 23).

Sound transmission through a medium, such as colloidal suspensions, porous materials, magneto-rheological medium and nanofluids, has also been a subject of great interest in recent years (24). The anomalous behaviour of the ultrasonic velocity in sintered TiO2 nanofluids provides information about the pore size and shape of nanoparticles (25). The most significant application of nanofluids is their use as heat transfer fluids. The main goal of nanofluids is to attain the highest possible value for thermal conductivity at the smallest possible concentrations of nanoparticles (26). There exists a new class of nanofluids having very low heat transfer rate which are used for cooling to maintain the desired performance and reliability of machines, microelectronic devices and optical instruments in the microelectronics and transportation industries (2729). Nanofluids have been extensively explored for use in many applications. These include cooling a new class of super powerful and small computers and other electronic devices for use in military systems, aeroplanes or spacecraft as well as for large-scale cooling. Al2O3–water nanofluids have been used to maintain a high temperature gradient in thermoelectrics that convert waste heat to useful electrical energy (30). Metal oxide nanoparticle-based nanofluids have been investigated to enhance energy efficiency in a heating, ventilation and air conditioning (HVAC) system to give major environmental benefits (29). Recent development suggests that these nanofluids can be utilised to enhance heat transfer from solar collectors to storage tanks and to increase energy density, making them potential candidates in the renewable energy industry. Other projected applications of nanofluids include sensors and diagnostics that instantly detect chemical warfare agents in water or water- or foodborne contamination. Iron oxide based nanofluids have shown great promise in biomedical applications such as cooling medical devices, cancer treatment and drug delivery (31).

One very important application of nanofluids is in heat transfer systems. Assorted studies have been carried out on the heat transfer enhancement of nanofluids and an appreciable enhancement has been found in the thermal conductivity correlated to the base fluid. Murshed et al. (13) measured the TCE of nanofluids by dispersing TiO2 nanoparticles in the matrix of ethylene glycol. They observed 18% TCE at 5 vol%. Duangthongsuk et al. (14) have done a similar study in water-based nanofluid by dispersion of TiO2 nanoparticles at 2 vol% and reported 7% TCE. Khedkar et al. (32) measured the TCE in TiO2 nanoparticles with ethylene glycol as base fluid. They reported 19.52% TCE at 7.0 vol% concentration of nanoparticles. Angayarkanni et al. (33) measured the TCE in TiO2 nanoparticles with water as base fluid. They reported 15.1% TCE at 4.0 vol% concentration of nanoparticles. Other metallic oxide nanoparticles have also been used for preparation of nanofluids. Beck et al. (30) determined the thermal conductivity of Al2O3/ethylene glycol nanofluids and reported a maximum TCE of up to 16.3% for 3.0 vol% concentration. Khedkar et al. (34) measured the temperature-dependent enhancement of thermal conductivity in CuO + water with different concentrations. They reported 32.3% TCE at 7.5 wt% concentration. Esfe et al. (35) measured the TCE in MgO nanoparticles with ethylene glycol + water (40:60 wt%) as base fluid. They reported 34.43% TCE at 3.0 vol% concentration of nanoparticles. Li et al. (36) determined the thermal conductivity of ZnO-ethylene glycol nanofluids and they reported the maximum TCE of nanofluid up to 13.0% for 2.4 vol% concentration. Murshed et al. (13) measured the TCE of nanofluids by dispersing CuO nanoparticles in the matrix of ethylene glycol. They observed 21% TCE at 2 vol%. All these measurements have been reported at higher temperature and higher volume fraction.

In the present work, we synthesised TiO2 nanoparticles through the chemical route and characterised by XRD, TEM, SEM-EDX and UV-vis spectroscopy techniques. After synthesis, the TiO2 nanoparticles were suspended in ethylene glycol as carrier fluid with the help of an ultrasonicator with different sonication times and nanoparticle concentrations to prepare TiO2-ethylene glycol nanofluids. The thermal conductivity measurements were performed for 0.2 wt%, 0.5 wt% and 1.0 wt% nanoparticle loaded nanofluids using a TPS-500 S Thermal Constants Analyser (Hot Disk, Sweden). Ultrasonic velocity and particle size distribution (PSD) measurements were done for the ultrasonic characterisation of the prepared nanofluids. The possible mechanisms of enhancement in thermal conductivity, ultrasonic velocity and PSD of nanoparticles in nanofluids are discussed. The reported data and their analysis suggest potential applications in industries associated with heat transfer management.

2. Experimental Details

2.1 Synthesis of Titania Nanoparticles

TiO2 nanoparticles were successfully synthesised by a simple sol-gel method (37) using Ti[OCH(CH3)2]4, generally referred to as titanium tetra-isopropoxide (TTIP), as a precursor purchased from Sigma-Aldrich Company (USA) with purity of 97%. Titanium(IV) isopropoxide was dropped slowly into the mixed solution of distilled water and ethanol in the ratios of 1:4:1 (TTIP: water: ethanol). The solution was stirred continuously for 1 h at room temperature to obtain a white slurry. HNO3 was used to adjust pH value in the range 2–3. The white slurry mixture was dried at 120°C for 3 h on a hot plate; the dried powder was sintered at 450°C for 3 h. Finally, we obtained the required TiO2 nanoparticles. The flow chart of synthesis of TiO2 nanoparticles is given in Figure 1.

Fig. 1

Flow chart showing the synthesis of TiO2 nanoparticles

The synthesised sample of TiO2 nanoparticles were analysed with XRD pattern using a SmartLab® X-ray diffractometer (Rigaku Corporation, Japan) (with λ = 1.5406 Å CuKα radiation) operating at 40 kV, 30 mA and at room temperature. The XRD patterns were used to determine the crystallite size, lattice parameter and phase identification. The structural and morphological analysis of TiO2 nanoparticles were done by HR-TEM and the selected area electron diffraction (SAED) pattern using the model TecnaiTM G2 F30 field emission gun transmission electron microscope (FEI Company, USA) operating at 200 kV accelerating voltage with resolution point:0.17 Angstrom line:1.24 Å and magnification 1500 LM to 520 kx. Tescan MAIA3 field emission scanning electron microscope (Tescan, Czech Republic) operating at 12.0 kV and magnification 21.4 Kx was used for SEM-EDX analysis of the morphology and average particle size of the TiO2 nanoparticles. The UV-vis absorption spectrum was recorded using Shimadzu UV-2330 spectrometer (Shimadzu Corporation, Japan) in the range 200–700 nm. The UV-vis spectrum was used to determine direct energy band gap of the TiO2 nanoparticles.

2.2 Preparation of Titania-Ethylene Glycol Nanofluids

TiO2-ethylene glycol nanofluids were prepared at different concentrations, 0.2 wt%, 0.5 wt% and 1.0 wt% of TiO2 nanoparticles. When TiO2 nanoparticles are added to the ethylene glycol base fluid, the nanoparticles produce a sediment within a few minutes because they remain in clusters without being dispersed. For the uniform dispersion of nanoparticles in the base fluid, we used an ultrasonic homogeniser VC 505 (Sonics & Materials Inc, USA) working at 20–40 kHz, 500 W.

3. Results and Discussion

3.1 Structural Analysis

The crystal phases of the synthesised TiO2 nanoparticles were determined by XRD patterns as shown in Figure 2. The obtained peaks in the diffraction pattern are identified with the JCPDS Card No. 88-1175. The interplanar spacing has been calculated using Equation (i):


where λ represents the wavelength of CuKα (1.5406 Å) radiation, θ is the angle between incident beam and the reflection lattice planes and n = 1 is the order of the XRD spectra. The highest peak is observed at 2θ = 25.4° which was indicated to plane (101) and d spacing corresponding to this peak is 3.12 Å. The other peaks in XRD pattern are observed at 2θ = 27.6°, 37.9°, 48.2°, 54.1°, 55.1°, 62.8°, 69°, 70.4°, 75.1° and 82.8° correspond to the (110), (004), (200), (105), (211), (002), (116), (112), (215) and (312) planes of TiO2 nanoparticles and d spacing are calculated as 2.83 Å, 2.43 Å, 1.88 Å, 1.69 Å, 1.66 Å, 1.47 Å, 1.35 Å, 1.33 Å, 1.26 Å and 1.16 Å, respectively. The intensity of the obtained peaks indicates the well-formed crystalline nature of the sample. The average crystallite size has been computed with Scherrer’s equation (Equation (ii)) (38):


where βhkl represents the full width at half maxima (FWHM) and K is the Scherrer constant. From this formula, the calculated average crystallite size of the given sample is approximately ~23 nm.

Fig. 2

XRD pattern of the powder sample of TiO2 nanoparticles

3.2 TEM, SEM and EDS/EDX Analysis

The TEM image of a crystalline sample is shown in Figure 3(a). The average particle size of the TiO2 nanoparticles ranged from 20–26 nm as shown in the histogram (Figure 3(b)). The SAED pattern in Figure 3(c) shows principally 10 rings which are ascribed to (101), (110), (103), (004), (111), (200), (105), (211), (002) and (116) planes, respectively. These planes are consistent with the XRD results. The d spacings are in agreement with the tetragonal structure of TiO2 nanoparticles (JCPDS Card No. 88-1175). For the structural analysis TiO2 nanoparticles were also examined by HR-TEM as shown in Figure 3(d). The crystalline nature of the nanoparticles is visible in the HR-TEM micrograph. The lattice spacing 0.31 nm and 0.28 nm corresponds to (101) and (110) planes respectively. The size and morphology of the TiO2 nanoparticles were also determined using SEM. Figure 4 shows typical SEM images of TiO2 nanoparticles. The SEM image shows random distribution of TiO2 nanoparticles having sizes in the range 18–26 nm. In Figure 4, there is a soft agglomeration of the nanoparticles: isolated particles are connected to each other by attractive physical interactions like Van der Waals force. The agglomeration of nanoparticles in the base fluid probably affects the thermal conductivity performance of the nanofluids. Agglomeration of nanoparticles affects the Brownian motion of the nanoparticles resulting in a decrease in thermal performance of the nanofluids. To remove agglomerations of nanoparticles in the base fluid, a sonication process has been used to break the intermolecular interactions. The EDX spectrum (Figure 5) of the TiO2 nanoparticles provides information about the constituent components of our sample, which contains titanium and oxygen. The high intensity peaks for titanium and oxygen justifies that the sample contains mainly TiO2.

Fig. 3

(a) TEM micrograph; (b) PSD; (c) SAED pattern; (d) lattice spacing HR-TEM of the TiO2 nanoparticles

Fig. 4

SEM micrograph of TiO2 nanoparticles

Fig. 5

EDX spectrometry of TiO2 nanoparticles

3.3 UV-Vis Spectra Analysis

The UV-vis absorption spectrum at room temperature of TiO2 nanoparticles has been recorded in the wavelength range 200–700 nm and is shown in Figure 6(a). It is obvious from the UV-vis absorption spectrum that the peak observed at 315 nm represents a blue shift compared with its bulk counterpart. This indicates that the particle size of the TiO2 nanoparticles has been reduced (3840). The optical absorption of the TiO2 nanoparticles is analysed by Equation (iii):


where Eg represents the optical band gap of nanoparticles, B is a constant, α is the optical absorption coefficient of the nanoparticles. The exponent m depends on the nature of the transition, m = 1/2, 2, 3/2, 3 for allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions respectively. Figure 6(b) shows the Tauc plot of TiO2 nanoparticles, a satisfactory fit is obtained for (αhν)2 vs. hν indicating the presence of a direct band gap. The optical energy gap of the TiO2 nanoparticles has been determined as 3.28 eV by extrapolating the linear portion of this plot at (αhν)2 = 0.

Fig. 6

(a) UV-vis absorption spectrum of sample TiO2; (b) the optical band gap calculation plot (αhν)2 vs. of TiO2 nanoparticles

3.4 Thermal Conductivity Measurement

The thermal conductivity of the nanofluids was measured by using a Hot Disk TPS-500 S thermal constant analyser. The Hot Disk TPS-500 S is the newest transient plane source (TPS) thermal constants analyser. The TPS technique has been used to determine the thermal conductivity of a nanofluid. The temperature dependent thermal conductivity of the TiO2-ethylene glycol nanofluids is plotted in Figure 7(a) at 0.2 wt%, 0.5 wt% and 1.0 wt%. The results show that the thermal conductivity of TiO2-ethylene glycol nanofluids increases with concentration of TiO2 nanoparticles. The thermal conductivity exhibits a slow increase for 0.2 wt% nanofluids while it shows relatively fast increase for 0.5 wt% and 1.0 wt% nanofluid in the temperature range 20–80°C. At 20°C, the value of thermal conductivity of pure ethylene glycol is 0.285 W mK−1 and it has been increased to 0.314 W mK−1 for 1.0 wt% concentration of TiO2 nanoparticles in ethylene glycol base fluid. The expression of TCE is given by Equation (iv) (41):


where TCnf and TCbf are the thermal conductivity of nanofluid and base fluid respectively.

Fig. 7

(a) Thermal conductivity of pure ethylene glycol and TiO2+ethylene glycol nanofluids with 0.2 wt%, 0.5 wt% and 1.0 wt% loading of TiO2 nanoparticles at different temperatures; (b) TCE of TiO2+ethylene glycol nanofluids with 0.2 wt%, 0.5 wt% and 1.0 wt% loading of TiO2 nanoparticles at different temperatures

A number of investigators have developed models for determining the thermal conductivity of nanofluids containing spherical particles. They only consider the effect of volume fraction of the particles. However the thermal conductivity of nanofluids depends on various factors such as size, shape, volume fraction of the suspended particles as well as temperature of suspensions. A few models also propose that the TCE is due to the ordered layering of liquid molecules near the solid particles (42, 43).

In addition to describing the TCE in nanofluids, we consider the effect of three possible mechanisms for heat transfer in nanofluids: (a) translational Brownian motion, (b) the existence of an interparticle potential and (c) convection in the liquid due to the Brownian movement. In the low temperature region, the mean free path due to the collision of nanoparticles increases and leads to TCE due to Brownian particle (KBrownian) as given as Equation (v):


where ϕ is the volume fraction, CN is the heat capacity per unit volume of the nanoparticles, l is the mean free path and VN is root mean square velocity of the particles.

This model explains the individual effect of temperature on the TCE in nanofluids with the help of Brownian motion but does not consider the effect of surface functionality and particle loading of nanoparticles. To overcome the shortcomings of this model, Prasher et al. (44) presents an order-of-magnitude justification to show a local convection effect caused by the Brownian movement of the nanoparticles. Based on the Brownian motion induced convection effect from multiple nanoparticles, the model of Prasher et al. for the TCE ratio of a nanofluid is given in Equation (vi):


where K is the thermal conductivity of nanofluids, Kf is the thermal conductivity of fluids, A and m are the best fit constants, and should be same for different experimental data for a particular fluid. Re and Pr are Reynolds and Prandtl numbers respectively (Equation (vii)):


where dN is the particle diameter, Rb is the interfacial resistance (Equation (viii)):


The Reynolds number (Re) is based on the root-mean-square velocity (νN) of a Brownian particle defined as Equation (ix) (45, 46):


where ρN is the density of the particles, kb is the Boltzmann constant and T is the temperature in Kelvin scale.

Figure 7(b) shows the variation of the TCE with temperature ranging from 20–80°C. It is clear from Figure 7(b) that the enhancement in thermal conductivity of TiO2-ethylene glycol nanofluids is achieved with increasing concentrations of nanoparticles and temperatures. At 20°C, we observed 3.3% to 6.5% and 11.2% TCE on 0.2 wt%, 0.5 wt% and 1.0 wt%, and at 80°C, the TCE becomes 9.9%, 18.3% and 23.8% for 0.2 wt%, 0.5 wt% and 1.0 wt% respectively for TiO2-ethylene glycol nanofluids. This enhancement is due to better uniformity and stability of suspensions. It has been found that ultrasonication increases the stability and uniformity of the nanofluids. The achieved values of TCE are higher than any of the results reported previously for TiO2-ethylene glycol or TiO2-water based nanofluids (13, 14, 30, 31) at such small concentrations. In preparation of the nanofluids, we used very small amounts of nanoparticles, so the fluidic properties of the liquid are almost unaffected, allowing for the easy flow of liquids and better transfer of heat. As the temperature increases, the TCE in the TiO2-ethylene glycol nanofluids may be attributed to Brownian motion of nanoparticles. It is obvious from Equation (ix) that the root-mean-square velocity (νN) of a Brownian particle depends upon particle diameter. If the particle diameter is small, root-mean-square velocity of a Brownian particle is large. Since the synthesised nanoparticles are small in diameter (approximately 22 nm) this results in the increase of Brownian motion, causing convection which in turn increases the thermal conductivity of the nanofluids. The high TCEs are probably due to the small size of nanoparticles because as the particle size decreases, the surface-to-volume ratio of particles increases, which can lead to enhanced thermal conductivity of nanofluids.

3.5 Determination of Ultrasonic Velocity using Interferometric Technique

The ultrasonic velocity in nanofluids was measured using an ultrasonic interferometer (model nanofluid-10X, Mittal Enterprises, India) at 3 MHz frequency in temperature range 20–80°C. The measured ultrasonic velocity in ethylene glycol matrix and three nanofluids samples containing 0.2 wt%, 0.5 wt% and 1.0 wt% of TiO2 in temperature range 20–80°C are shown in Figure 8. It is obvious from Figure 8 that the ultrasonic velocity in the nanofluids increases with the temperature. The plot also indicates that the ultrasonic velocity in the nanofluids is larger than that of pure ethylene glycol matrix (1410 m s−1) at 20°C and the velocity increases with the particle concentration (1430 m s−1) for 1.0 wt% loading at the same temperature of 20°C.

Fig. 8

Ultrasonic velocity vs. temperature in different samples of TiO2+ethylene glycol nanofluid and pure ethylene glycol

If we consider (ρm,ρs) and (km,ks) are the density and the compressibility of fluid and suspended particles respectively, B and ϕ are the bulk modulus and the particle volume fraction; then the effective density (ρeff) and compressibility (keff) of the suspension becomes as Equation (x) (4749):


The ultrasonic velocity (V) in a medium is given by Equation (xi):


where B, ρ and k represent the bulk modulus, density and compressibility of the medium respectively. λ and μ are the material dependent quantities known as Lamé moduli or Lamé coefficients. The compressibility and density of a fluid medium are changed by the dispersion of nanoparticles and are the function of the particle volume fraction. From Equation (x), it is clear that the evaluation of the effective bulk modulus and compressibility of the suspension is performed with calculation of effective Lamé moduli, which depends on particle volume fraction of suspended particles.

It is obvious from Equations (x) and (xi) that the bulk modulus and change in density of the nanoparticles suspension as a function of volume fraction causes an enhancement in the ultrasonic velocity. An increase in the wave velocity with increase in the particle concentration of given nanofluids indicates that there is positive change in the bulk modulus and density of the nanofluids. It may be predicted that the comparative change in the density with respect to bulk modulus is small. As the particle concentration in nanofluids increases, the compressibility of the given matrix decreases. A strong cohesive interaction occurs among the molecules after dispersion of TiO2 nanoparticles in the ethylene glycol matrix. Thus for the TiO2 nanofluids, the ultrasonic velocities are larger in comparison to the ethylene glycol matrix and increase with the nanoparticle concentration.

In the low frequency region, the velocity in nanofluids is independent of particle size (49, 50). Here all the nanofluids have been prepared with nanoparticles fabricated at low evaporation rate and velocity of the ultrasonic wave is measured at different temperatures and low frequency (3 MHz). Thus it was concluded that the temperature dependent velocity at low frequency in the nanofluids depends only on the particle concentration. At low frequency, the ultrasonic velocity in a nanofluid is a quadratic function of temperature (Equation (xii) (51):


where V0 is the ultrasonic velocity at 0°C, V1 and V2 are the absolute temperature coefficients of velocity and T is the temperature difference between experimental and initial temperature (0°C). The first and second terms in Equation (xii) are in good agreement for a simple liquid system, but the third nonlinear term is caused by non-linear change in bulk modulus and density of the nanofluid system with temperature.

3.6. Particle Size Distribution in Titania+Ethylene Glycol Nanofluid by Acoustical Particle Sizer

The acoustic particle sizer APS-100 (Matec Applied Sciences, USA) was used to examine the PSD in the nanofluids. The APS-100 works on Epstein and Carhart theory (52) and is mainly based on the ultrasonic spectroscopic method. The APS-100 computes the sound attenuation (dB) per unit length (cm) over the 1–100 MHz frequency range in particle-liquid suspensions with high precision. This attenuation spectrum can be converted to PSD data. According to Epstein and Carhart theory (52), the attenuation of the ultrasonic wave in a nanofluid can be understood with the understanding of the thermal wave length (; KS, ρS and CS: thermal conductivity, density and specific heat of the dispersed particle: ω ; frequency of the wave) and the viscous wave length (; η: viscosity of the matrix). When the viscous wave length is comparable to particle radius (r), the viscous loss is a prominent cause behind the ultrasonic attenuation; while the viscous drag, scattering and thermal losses are effective when the thermal wave length λTr. The expressions for the ordinary viscous dissipation V), the viscous drag loss VD) (47, 50) of the sound waves are given as Equation (xiii) and Equation (xiv):




where ηd and ηV represent the dynamic and the volume viscosities of the nanofluid, is the wave number, . Biwa (53) calculated the change in the ultrasonic attenuation with respect to volume fraction caused by scattering at microscale in low frequency limit. The expression to compute the ultrasonic attenuation is given as Equation (xv) (53):


where γsca represents the scattering cross-section which depends on the frequency of the ultrasonic wave, particle size, bulk modulus and density of the base/carrier fluid and suspended particles. The thermal attenuation is caused by temperature variation produced by propagation of the sound waves in different components of suspension. The thermal loss mainly depends on the frequency and particle size. The particle size has been obtained by APS-100 in range of 19 nm to 24 nm as visualised in Figure 9. It has been confirmed from Figures 3(a) and 9 that the PSD obtained by APS-100 is in good agreement with that obtained by the TEM micrograph. Ultrasonic spectroscopy is sensitive to particles with radius between about 10 nm to 1000 mm. The maximum particle concentration which can be analysed varies between about 1 wt% to 50 wt% depending on the nature of the system. On the other hand, the technique is unsuitable for analysing dilute suspensions i.e., particle concentrations below about 1 wt%. In the present study (Figure 9) the weight percentage of TiO2 is 1 wt%. Systems with different weight percentages of TiO2 show the same PSD because nanoparticles are dispersed in base fluid with the same technique and the same ultrasonication time.

Fig. 9

PSD (%) of TiO2+ethylene glycol nanofluid using APS-100

4. Conclusions

In this investigation, we have successfully synthesised TiO2 nanoparticles by a simple sol-gel method. The structural and morphological characterisation of synthesised TiO2 nanoparticles was performed by XRD and HR-TEM. The average crystallite size was found to be ~23 nm using Scherrer’s formula. The average particle size was observed by TEM micrograph and found to be in the range 20–26 nm. The optical energy gap of the TiO2 nanoparticles was determined using Tauc plot and found to be 3.28 eV. Ultrasonic spectroscopy was used to determine the size of the nanoparticles and their distribution in the matrix was established. The results were in good agreement with the more costly TEM method. TPS was used to measure the thermal conductivity of the TiO2+ethylene glycol nanofluids. The thermal conductivity of nanofluids increases with increase in temperature as well as particle loading. TCE of 24% was observed for 1.0 wt% loading of TiO2 nanoparticles at 80°C. Thermal conductivity data are very important for advanced heat transfer management systems in many industrial applications. The temperature and concentration dependence of the ultrasonic velocity in nanofluids was also measured. The ultrasonic velocity in nanofluids mainly depends on the particle loadings.

Thus, the anomalous enhancement of the thermal conductivity of nanofluids can be used in coolant technology and in heat transfer management systems. Ultrasonic techniques are an efficient tool to characterise nanofluids, providing useful information about the emerging properties of exotic materials like nanofluids along with their microstructural features.


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The Authors

Mohit Gupta obtained his MSc in Physics with Condensed Matter Physics and received his PhD degree from the Physics Department, University of Allahabad, India in the field of Ultrasonics and Materials Science. He has been selected as a Physics Lecturer in the Technical Education Department (TED) Uttar Pradesh, India. His research interests are in the field of ultrasonic non-destructive characterisation of nanomaterials.

Devraj Singh is Professor and Head in Department of Physics and Director of the Prof. Rajendra Singh (Rajju Bhaiya) Institute of Physical Sciences for Study and Research, Veer Bahadur Singh Purvanchal University, Jaunpur, India. He obtained his DPhil in Science at the University of Allahabad in 2002. His research and publication interests include the mechanical, thermophysical and ultrasonic characterisation of condensed and advanced materials. He is a Fellow of the Ultrasonics Society of India and a life member of a number of scientific societies. He has 103 research articles published in national and international journals of ultrasonic and materials science and 24 published books to his credit.

Shakti Pratap Singh obtained his MSc in Physics with Spectroscopy and received his PhD degree from the Physics Department, University of Allahabad in the field of Ultrasonics and Materials Science. Since June 2020 he is working as Postdoctoral Fellow in the Department of Physics, Prof. Rajendra Singh (Rajju Bhaiya) Institute of Physical Sciences for Study and Research, Veer Bahadur Singh Purvanchal University, Jaunpur, India. His research interests are in the field of thermal and ultrasonic non-destructive characterisation of nanomaterials for industrial applications.

Ashish Mathur received a PhD degree from NIBEC, University of Ulster in 2011; MTech (Nanotechnology) from Amity Institute of Nanotechnology, Noida, India, in 2007 and MSc (Electronics) from Chhatrapati Shahu Ji Maharaj (CSJM) University, Kanpur, India, in 2005. Currently he is working as Associate Professor in Department of Physics, School of Engineering, IPES Bidholi Campus, Dehradun, Uttarakhand - 248007, India. He also worked as a Research Associate at NIBEC, University of Ulster, Jordanstown campus from July 2010 to December 2012. His research interest includes microfluids, sensors, carbon nanostructures, micro-electromechanical systems and nano-electromechanical systems.

Shikha Wadhwa received her PhD degree in Material Science from the University of Ulster, UK, in 2011. She then worked as a Research Associate at the Nanotechnology and Integrated Bioengineering Centre (NIBEC), University of Ulster (2011–2012). She is currently working as Assistant Professor at Department of Chemistry, School of Engineering, UPES Bidholi Campus, Dehradun, Uttarakhand - 248007, India. She has broad experience in the field of material science with special emphasis on nanomaterial and composite synthesis for a wide range of applications such as photocatalysis, antibacterial properties, electrochemical and sensing applications.

Aashit Kumar Jaiswal obtained his PhD from the Physics Department, University of Allahabad. He is currently working as Assistant Professor in Department of Physics, H.N.B. Govt. P.G. College, Naini, Prayagraj, India. His research interests are thermal conductivity, phonon transport in nanomaterials and non-destructive techniques for characterisation of materials. He has published several important studies on heat transfer phenomena of nanofluids in internationally reputed journals.

Dharmendra Kumar Singh obtained his PhD from University of Allahabad. He is Assistant Professor and Head of Physics Department at Shivharsh Kisan Post-Graduate College, Basti, India. His area of research includes synthesis and characterisation of optical and giant magnetoresistance nanomaterials.

Professor Raja Ram Yadav is Ex-Vice-Chancellor V.B.S. Purvanchal University and Professor of Physics at the Department of Physics, University of Allahabad. His research interests are in the non-destructive ultrasonic and thermal characterisation of nanomaterials, lyotropic liquid crystalline materials, intermetallics and semiconductors; the development of nanomaterials for biomedical applications; and theoretical calculations of nonlinear elastic and ultrasonic properties of crystalline materials. Professor Yadav successfully coupled ultrasonics with nanoscience and technology recognised as the new area of ‘Ultrasonics in Nanoparticles-Liquid Suspensions’ in the field of acoustics. He has 138 research articles.

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